the thing is tough, I'd rather take the _extra parameter all away together (Lsyncd 3.0 in preparation, albeit I admit that project got a little stalled) will not have an _extra parameter.
The reasoning is, I see in the issue reports people putting stuff into it they really shouldn't. I wrote in the documentation, the _extra parameter is ment as workaround for people knowing what they're doing. However, I think, people that know what they do can simply overload the necessary functions thus making it "a patch".
If you're missing specific ssh parameters, I'll welcome patches for those.
Kind regards, Axel